1. Field of the Invention
The present invention relates to a material preformed to a required shape for use in fiber reinforced plastics (hereinafter referred to as FRP) in order to reinforce a matrix resin of an FRP when the FRP is formed.
2. Description of the Prior Art
Various methods for forming FRP are well known, and generally what type of method should be adopted is decided in accordance with required properties and output of production of an FRP. For instance, in the case where relatively high mechanical properties are required for an FRP but the number of products is small, an autoclave molding process is often adopted for the FRP. In the case where high mechanical properties are not required for an FRP but the number of products is relatively great, a hand lay up molding method is often adopted for the FRP. Further, a resin injection molding method for the case where the number of products is 500-1000 as a middle class of production scale, and a continuous pultrusion molding method where the number of products is more than the above number are generally adopted respectively. However, in the case where fibers, such as carbon fibers, which have a high strength and a high elastic modulus (Young's modulus) but are expensive are employed as reinforcing fibers for an FRP, the resin injection molding method and the continuous pultrusion molding method are not so often adopted. The reason why the methods are not so often adopted is that the arrangement or distribution of the reinforcing fibers tends to be disturbed when the FRP is formed, thereby decreasing the reinforcing effect according to high characteristic properties of carbon fibers etc., as described hereinafter. Therefore advantages according to use of carbon fibers etc. decrease.
To explain the above problems in detail, a resin injection molding method is a method wherein, after dry sheet-like substrates composed of reinforcing fibers are laminated and charged into a cavity defined by male and female molds, a resin is injected into the cavity and the resin is impregnated into the substrates. The resin is then cured. In this method, the preformed material composed of laminated substrates tends to get out of shape, particularly when the substrates are laminated to a complicated shape. Moreover, there is a problem that the substrates are moved by the pressure applied when the resin is injected, thereby changing the laminated state of the substrates. This causes a disturbance of the arrangement or the distribution of the reinforcing fibers in the preformed material.
A continuous pultrusion molding method is a method wherein sheet-like substrates are drawn from creels, a resin is impregnated into the drawn and laminated substrates, the substrates into which the resin is impregnated are shaped and the resin is cured by passing the substrates through a heated die, and the obtained FRP is then continuously drawn to a puller. Also in this method, a disturbance of the orientation or the distribution of the reinforcing fibers in the preformed material tends to occur on account of the deflection of the substrates due to the weight of the resin or the wrinkles of the substrates generated during the process, particularly after the resin is impregnated.
On the other hand, the following preformed material is disclosed in page 402 of Plastic Processing Technology Handbook published on July 30, 1975 by Nikkan Kogyo Shinbunsha of a Japanese company. The preformed material is formed by laminating substrates composed of reinforcing fibers to a required shape and bonding the substrates to each other with chips made by cutting a substrate of reinforcing fibers in which a resin is impregnated or with powders of a thermoplastic material. However, this forming method also has a problem that the characteristic properties of carbon fibers etc. cannot be sufficiently obtained.
Therefore, in the case where the chips are employed, non-uniform FRP tends to be obtained because the volume fraction of fibers contained in the portion of the FRP where the chips are becomes locally higher than in other portions. Moreover, in the case where the substrates are composed of long fibers or filamentary yarns, since the short fibers of the chips exist between the substrates, stress concentrates at the portions where the chips are and the fracture of the FRP tends to begin from the portions relatively early.
On the other hand, in the case where the thermoplastic powders are employed, since it is difficult to distribute the powder uniformly on the substrates, fairly large non-uniformity occurs in accordance with the distribution of the powders. Accordingly, in order to provide the laminated substrates with a necessary bonding strength required for a preformed material, the amount of the powders should be relatively large. Since the powders do not constitute the matrix of the FRP, however, the increase in the amount of the powders deteriorates the properties of the FRP.
Furthermore, JP-A-58-39442 discloses a preformed material formed by disposing webs, composed of short heat-melting fibers and formed as a mat, a strip or a grid between a mat of glass fibers and a woven fabric of glass fibers, and bonding the mat and the woven fabric by melting the webs. However, this preformed material has problems similar to the problems in the aforementioned materials.
Therefore, in the case where the web is formed as a mat wherein the short fibers are arranged in random directions on the surface of the mat, since such a mat ordinarily has a large non-uniformity in accordance with the distribution of the short fibers, it becomes necessary to increase the amount of webs used for providing a required bonding strength for a preformed material. Since the webs do not constitute the matrix of the FRP even if the webs melt, the increase in the number of webs causes a deterioration in the properties of the FRP. Moreover, with respect to a woven fabric, convex portions of the bending weaving yarns of the woven fabric are the most suitable portions for effective bonding. However, when the webs formed as a mat melt, the molten materials adhere not only to the convex portions but also to concave portions and mesh portions, that is, the portions where the weaving yarns do not exist. This causes a very great increase in the amount of the webs required and an adequate impregnation of a resin becomes difficult when an FRP is formed.
In the case where the webs formed as a strip or a grid are employed, the arrangement of the short fibers is usually performed so as to form straps. Since it is extremely difficult to adapt the pitch of the straps to the pitch of the convex portions which are the most suitable portions for bonding, not only an effective bonding becomes difficult but also the amount of the webs increases in order to obtain an adequate bonding strength for a preformed material. There occur similar problems even if the short fibers are arranged in fine yarn-like shapes.